SUMMARY Timing of food intake affects a variety of pathophysiological systems. The Western diet, which is high in salt, also contributes to excess morbidity and mortality related to obesity and hypertension. Nocturnal hypertension frequently occurs in obesity and is recognized as an important consequence of hypertension risk, yet the mechanisms involved in this phenomenon are poorly understood. Experimental data from our group have shown that timing of sodium intake impacts diurnal patterns of sodium excretion. Further, we recently reported that high salt intake causes a shift in expression of circadian control genes in the kidney. Additional studies demonstrate that obese animals have an impaired response to a natriuretic stimulus. Given the established contribution of high salt intake to obesity-dependent hypertension, particularly, nocturnal hypertension, we hypothesize that the time of day for salt intake impacts (1) blood pressure rhythms and urinary sodium excretion and (2) circadian timing of factors responsible for blood pressure regulation and cardiometabolic health in obese individuals. Because of the very high prevalence of nocturnal hypertension and salt-sensitivity in black adults, we will conduct a randomized, cross-over feeding study of 55 obese black adults with non-dipping sleep blood pressure. These studies will address two aims. The first aim will test the hypothesis that limiting high salt intake prior to sleep increases day-night differences in blood pressure, improves timing of urinary sodium excretion, and improves metabolic risk factors. We will monitor 24-hour blood pressure by ambulatory blood pressure monitoring to determine the role of timing of sodium intake on diurnal blood pressure patterns. Day- and night- time sodium excretion will be used to determine whether improvements in blood pressure are mediated by enhanced sodium excretion during the day. We will also assess the effects of timing of sodium intake on lipids, leptin, adiponectin, insulin sensitivity, inflammatory cytokines, and immune cell activation over 24 hours. The second aim will test the hypothesis that limiting high salt intake prior to sleep preferentially improves rhythmicity in peripheral vs. central circadian clock factors linked to renal sodium handling. Circadian measures of plasma cortisol, dim light melatonin onset, and core body temperature (telemetry) will be used to assess the phase and amplitude of the core circadian clock. Circadian measures of peripheral clock genes in buccal cells and peripheral blood monocytes will be used to determine the phase and amplitude of the peripheral clock. The proposed hypothesis-driven studies will determine how timing of sodium intake affects diurnal blood pressure and circadian timing of factors responsible for blood pressure control and metabolic health, with the ultimate goal of identifying novel strategies to treat nocturnal hypertension and metabolic disease in obesity.